1. Field of the Invention
The present invention generally relates to an image processing apparatus, image forming apparatus, and image processing method to process image data corresponding to increment regions, in order to form an image in increment regions of a recording medium, with a relative movement (relative scanning) performed multiple times between a recording head and a recording medium.
2. Description of the Related Art
As an example of a recording method using a recording head having multiple recording devices, an inkjet recording method that discharges ink from individual recording devices and forms dots on a recording medium has been used. With a serial-type inkjet recording apparatus, an image is formed by repeating recording main scanning, which scans recording heads that discharge ink, and conveying operations that convey the recording medium in a direction orthogonal to the recording main scanning. Such serial-type inkjet recording apparatuses can be manufactured to be relatively small and at relatively low cost, and therefore are widely used for personal use.
With a recording head wherein multiple recording devices are arrayed, discharge amount and discharge direction can be scattered between the recording devices. As a result of this scattering, density unevenness or stripes can occur on the image.
A multi-pass recording method is used as a technique to reduce such image distortions. With the multi-pass recording method, the image data to be recorded in incremental areas of the recording medium are typically segmented into image data corresponding to multiple scans, and by sequentially recording the segmented image data with the multiple scans, with intervening conveying of the recording medium, the image is completed. With such a multi-pass recording method, image distortions resulting from discharge scattering for each recording device can often be reduced. Consequently, an even and smooth image can be obtained. With such a multi-pass recording method, the greater the number of times of the multi-pass, i.e. the number of recording devices used to record one scanning line, the greater the advantages thereof are typically increased. However, for a greater number of multi-passes, the recording speed may become decreased. Thus, a general-use serial-type inkjet recording apparatus is often configured so that multiple recording modes with different numbers of times of multi-passes can be selectively executed.
In the event of performing such multi-pass recording, the image data to be recorded in unit areas may be segmented into image data corresponding to individual recording scans. Heretofore, such data segmenting has often been performed using a mask pattern, wherein recording permitting pixels (1) permitting dot recording, and non-recording permitting pixels (0) not permitting dot recording, are arrayed.
FIG. 13 is a schematic diagram illustrating an example of a mask pattern which can be used with two-pass multi-pass recording. The areas shown in black indicate recording permitting pixels (1) and the areas shown in white indicate non-recording permitting pixels (0). A mask pattern 1801 is used with the first pass recording scanning, and 1802 indicates a mask pattern used with the second pass recording scanning. Also, the 1801 mask pattern and the 1802 mask pattern may have a mutually supplemental relation.
By performing a logical AND operation between such a mask pattern and binary image data, the binary image data may be segmented into binary image data to be recorded with each recording scan. For example, as shown in FIG. 2, by segmenting the image data showing the dots to be recorded in the unit areas with the mark patterns shown in FIG. 13 (1801, 1802), segmented image data may be generated for both the first pass and the second pass. Thus, with the data segmenting method (mask segmenting method) performed using a mask pattern having a mutually supplemental relation, the binary image data corresponding to the different scans may also have an interpolating relation, whereby the probability that the dots recorded with different scans become overlaid may be relatively low. Therefore, a relatively high density resulting from high dot coverage can be realized, and additionally, favorable granularity can also be secured.
Note that even though such multi-pass recording is employed, there is increasing demand for images with even higher image quality, and density changes or density unevenness resulting from recording scanning increments or shifting in recording position (registration) in recording device array increments can remain. Shifting in the recording positions of the recording scanning increments or recording device array increments may result from fluctuations in the distance between the recording medium and discharge output face (between the paper), fluctuations in conveying amounts of the recording medium, and so forth.
For example, referring to FIG. 2, a case is considered wherein a plane of dots (indicated by single circles) recorded with the leading recording scan, and a plane of dots (indicated by double circles) recorded with the following recording scan, have each shifted by one pixel worth in one of either the main scanning direction or sub scanning direction. In this case, the single-circle dots recorded with the leading recording scan and the double-circle dots recorded with the following recording scan completely overlay one another, whereby an area of white paper is exposed, and image density is decreased. Even if the shift is not as great as an entire pixel worth, if the distance or overlaying amounts between adjacent dots changes, the coverage of dots on the white paper area may fluctuate, and this fluctuation in coverage invites fluctuation in the image density. Such fluctuations in image density may then be recognized as overall density unevenness.
Accordingly, as demand for ever higher quality images increases, a need remains for a processing method for image data at the time of multi-pass recording, which can counter recording position shift between planes that can occur along with fluctuations in various recording conditions. Hereafter, regardless of the reason for the fluctuations and whatever the recording condition, a resistance to density changes and density unevenness that occur from recording position shifts because of such fluctuations is called “robustness” in the present Specification.
Japanese Patent Laid-Open No. 2000-103088 discloses an image data processing method to increase the above-described robustness. According to this document, the fluctuations in image density that occur along with fluctuations in various recording conditions, result from the binary data corresponding to different recording scans being in a mutually complete interpolating relation. As understood from this document, if image data corresponding to different recording scans is generated so that the above supplemental relation is reduced, it is believed that excellent multi-pass recording can be realized. In order to do so, in the Japanese Patent Laid-Open No. 2000-103088, image data is segmented in the state of multivalued data before binarization, and the multivalued data is independently binarized after segmenting. Thereby, even if image data of different planes corresponding to different recording scans shifts with respect to one another, excessively large density fluctuations may not occur.
FIGS. 3A through 3I are diagrams to describe the data segmenting method disclosed in Japanese Patent Laid-Open No. 2000-103088. First, multivalued image data to be recorded in the unit area (see FIG. 3A) is segmented into the multivalued data to be recorded with the first pass (see FIGS. 3B and 3D) and multivalued data to be recorded with the second pass (see FIGS. 3C and 3E). Next, the various multivalued data are individually binarized (see FIGS. 3F and 3G), whereby binary data to be recorded with the first pass (see FIG. 3H) and binary data to be recorded with the second pass (see FIG. 3I) are generated. Lastly, ink is discharged from the recording heads according to the binary data. As shown in FIGS. 3H and 3I, the binary data of the first pass and binary data of the second pass generated as described above are not in a perfectly supplemental relation. Accordingly, there are portions where dots are overlaid (pixels with “1” existing at two planes), and portions where dots are not overlaid (pixels with “1” existing at one plane), between the first pass and the second pass.
FIG. 4 is a diagram showing a state of dots recorded according to the method disclosed in Japanese Patent Laid-Open No. 2000-103088 that are arrayed on the recording medium. In the diagram, a black circle 21 indicates a dot recorded with the first pass, a white circle 22 indicates a dot recorded with the second pass, and a shaded circle 23 indicates a dot recorded by overlaying the first pass and the second pass. In this example, the supplemental relation of the first pass and the second pass are incomplete, thus differing from the case in FIG. 2 wherein the supplemental relation is complete, and thus there are portions wherein two dots overlap and portions wherein dots are not recorded (white sheet area).
As with the case in FIG. 2, a case is considered wherein a dot recorded with the first pass and a dot recorded with the second pass, are shifted one pixel worth in either of the main scanning direction or sub scanning direction. In this case, if there is no position shifting, the dots in the first pass and the dots in the second pass that should not have been overlaid can be overlaid, and the dots 23 that should have been overlaid with no position shifting now are not overlaid. Accordingly, for an area having a certain width, the coverage of dots on the white sheet area does not exhibit as much fluctuation, and image density changes are relatively low. That is to say, with the method in the Japanese Patent Laid-Open No. 2000-103088, even if fluctuations in the distance between the recording medium (e.g., paper) and discharge opening face, and fluctuations in the amount of conveyance of the recording medium occur, accompanying image density fluctuations can be suppressed.
Further, as with Japanese Patent Laid-Open No. 2000-103088, Japanese Patent Laid-Open No. 2006-231736 discloses a technique whereby image data is distributed in multiple recording scans or multiple recording device rows while in the state of multivalued image data, while the distribution rate of such data may be varied based on the image positions. According to this document, the distribution rate can be changed linearly, cyclically, sinusoidally, or in composite waveform of high frequencies and low frequencies, whereby banding and color unevenness with the multi-pass recording method can be suppressed.
However, while the methods in Japanese Patent Laid-Open No. 2000-103088 and Japanese Patent Laid-Open No. 2006-231736 (hereafter referred to, for the sake of convenience, as the “multivalued data segmenting method”) are excellent in robustness as compared to the mask segmenting method, in some aspects they may also be inferior to the mask segmenting method. That is to say, the multivalued data segmenting method may tend to have a lower image density because coverage may be lower as compared to the mask segmenting method, and the method may have more of a poor granular feel because overlaying of dots may be high. Also, as the multiple data segmenting method may perform binarizing processing as many times as the multivalued data is segmented, the binarization processing load may be greater as compared to the mask segmenting method.
Thus, while the multiple data segmenting method may be superior to the mask segmenting method in some points, it may also be inferior thereto in other points, and thus multivalued data segmenting method is not uniformly used for all multi-pass modes. That is to say, with a multi-pass mode having different numbers of passes, images subject to main recording and conveyance errors may differ, and thus the time that is consumed for data processing may differ, and also the significance of the existence of the modes may also differ. Accordingly, the data segmenting method may be selected in accordance with the objective or significance of existence of the multi-pass mode. For example, with a mode with only a relatively few numbers of passes, sufficient conveying precision may not capable of being secured, and thus if the apparatus has relatively great density fluctuations according to conveying errors, employing a data segmenting method that is excellent for suppressing density fluctuations that accompany conveying errors for a low-pass mode may be one option. Conversely, in the case where quality of text or line drawings making up the main subject image with a mode having fewer passes is prioritized over robustness, one option may be to employ the mask segmenting method, which is effective for securing text and line drawing quality. In either case, the balance of the overall apparatus may be taken into consideration and the data segmenting method appropriate to the number of times of multi-passes may be selected.